In general, the blood plasma disappearance rate and the retention rate have been measured by a method of blood collection through use of indocyanine green (hereinafter referred to as ICG) serving as a specific dye. According to this method, an intravenous injection of ICG is given to a testee and a blood sample is taken three times after lapses of five, ten and 15 minutes from the injection. The blood serum is separated upon coagulation of a blood clot so that an absorbance at a wavelength of 805 nm is measured through a spectrophotometer to obtain ICG concentration values in the blood serum after the lapses of five, ten and 15 minutes relative to a previously obtained calibration curve showing the ICG concentration in blood vs. absorbance thereby to calculate the blood plasma disappearance rate and the retention rate. In recent years, a method of changing the quantity of the ICG injection has been used to measure the blood plasma disappearance rate several times thereby to obtain an index expressing a quantity of a hepatic cell function R.sub.MAX (removed maximal).
Japanese Patent Publication Gazette No. 58649/1985 has already proposed a method of measuring the blood plasma disappearance rate and the retention rate without performing blood collections. According to that method, light is applied through the body surface of an organism, which in turn transmits light of a wavelength having a high ICG absorption sensitivity and light of a wavelength having substantially no ICG absorption sensitivity. The respective quantities of transmitted light are measured to obtain the blood plasma disappearance rate and the retention rate of the light quantities as a function of time (dye disappearance curve).
In the aforementioned first method requiring the taking of blood samples, it is necessary to correctly measure the blood collection time after the injection of the dye. However, the time has not been accurately measured in an actual test, and the operation for such measurement has been complicated. Further, the testee has been subject to substantial mental and physical burdens caused by the repeated taking of blood samples. In addition, the index R.sub.MAX method for measuring the blood plasma disappearance rate several times by changing the quantity of the ICG injection requires taking blood samples more than ten times, whereby the burdens on the testee are further increased.
According to the second measuring method without blood collection as disclosed in Japanese Patent Publication No. 58649/1985, the output of a sensor actually attached to an organism fluctuates under the influence of such factors as a blood flow disturbance caused by compression applied to a blood vessel, vibration of the organism being the object of measurement, pulsation in the organism, change of the blood volume in the vital tissue (the blood volume in each part in the vital tissue is changed by merely vertically moving an arm) etc. As a result correct dye disappearance curve cannot be obtained.